Prepare the Peanut Butter Filling:
Place the peanut butter in a heat-safe bowl and microwave for 20-30 seconds, until it’s pourable but not too hot.
Line an 8-inch square metal pan with parchment paper, leaving an overhang for easy removal. Spread the peanut butter in an even layer in the pan. Freeze for 1 hour or until the peanut butter layer is solidified.
Once frozen, use the parchment to lift the peanut butter sheet out of the pan and return it to the freezer while you prepare the brownie batter.
Make the Brownie Batter:
Preheat the oven to 350°F (175°C). Line the square pan again with parchment paper, leaving an overhang for easy removal.
In a large microwave-safe bowl, combine the butter and chopped chocolate. Microwave in 30-second intervals, stirring between each interval, until fully melted and smooth.
Add the sugar to the melted butter and chocolate mixture, whisking vigorously until combined. Let the mixture cool until just warm.
Mix in the Wet Ingredients:
Whisk in the eggs, yolk, and vanilla extract, whisking for about 1 minute until well combined and slightly thickened.
Combine the Dry Ingredients:
Using a rubber spatula, gently fold in the flour, sifted cocoa powder, and salt until just combined, being careful not to overmix.
Assemble the Brownies:
Pour half of the brownie batter into the prepared pan and smooth the surface. Carefully place the frozen peanut butter sheet over the batter. Pour the remaining brownie batter on top, making sure the peanut butter layer is fully covered.
Bake the Brownies:
Bake in the preheated oven for about 30 minutes, or until the brownies are set but still slightly gooey in the center.
Cool and Serve:
Let the brownies cool in the pan for 30 minutes, then lift them out using the parchment paper. Allow to cool for an additional 30 minutes before slicing into squares.
Store in an airtight container at room temperature for up to 3 days or in the fridge for up to 5 days. For an extra fudgy texture, serve the brownies chilled.